1. Field of the Invention
The present invention relates to a gas sensor with a pH-sensitive FET transducer (hereinafter referred to as FET transducer) having a gate-insulated field-effect transistor structure.
2. Description of the Prior Art
Measurement of gas concentration such as carbon dioxide or ammonia is of importance in industrial application. Recently, in the medical and physiological fields, importance has been attached to measurement of gas concentration in a living body. For example, it is recognized that measurement of intracellular CO.sub.2 gas concentration provides information which is significant from a physiological view-point. In the medical field, continued measurement of monitoring blood CO.sub.2 concentration serves the purpose of the state of a patient who is anesthetized, is seriously ill or who is convalescing or provides an indication that a person is in a state of emergency. An extremely small CO.sub.2 sensor which is insertible into a cell or blood vessel is needed for the purpose of such a measurement.
A carbon dioxide sensor which has been employed for in the past for such purposes utilizes a miniaturized pH sensitive glass electrode which is shown schematically in FIG. 1. Known as Severinghaus-type CO.sub.2 sensor, this sensor consists of a pH-sensitive glass electrode 1, an Ag-AgCl reference electrode 2, an aqueous solution of sodium bicarbonate 3, and a gas permeable membrane 4. The sensor takes advantage of the fact that water-dissolved carbon dioxide dissociates into H.sup.+ and HCO.sub.3.sup.- ions.
When it is dissolved in water, a part of the CO.sub.2 is transformed into carbonic acid, the dissociation of which results in the formation of protons. ##EQU1## Therefore, the following relation holds between CO.sub.2 concentration in the solution and proton concentration: ##EQU2## Moreover, CO.sub.2 concentration in the solution is proportional to partial gas-phase CO.sub.2 pressure Pco.sub.2 as follows: ##EQU3## Use of these formulas, with -log [H.sup.+ ]=pH and log K=pK, leads to the relation known as the Henderson-Hasselbalch formula: ##EQU4## Here, if the aqueous solution contains no electrolyte that will produce bicarbonate ion, such as NaHCO.sub.3, or proton source other than CO.sub.2, the relation [H.sup.+ ]=[HCO.sub.3.sup.-] holds. Accordingly, ##EQU5## Under a constant temperature condition, both K and .alpha. are constant. Thus, formula (5) may be rewritten: ##EQU6## Where, in the solution, NaHCO.sub.3 is present excessively relative to CO.sub.2, [HCO.sub.3.sup.- ] is also constant. Then, from formula (4) is derived the following relation: ##EQU7## As is apparent from the above discussion, pH of the solution varies in proportion to the logarithm of partial CO.sub.2 pressure. In the light of this fact, the amount of CO.sub.2 in solution can be determined by means of a pH-sensitive electrode. In this connection, it is noted that, as can be seen from a comparison between formula (6) and formula (7), electrolytes such as NaHCO.sub.3 present in solution serve to double the proportional coefficient in proportional relation between pH and log (Pco.sub.2), that is, the output of the pH-sensitive electrode.
There are various kinds of gases, including CO.sub.2, that will dissolve in water to form proton ions and change the pH of the aqueous solution, as listed below. ##EQU8##
Quantitative analysis by means of a pH.sub.2 sensitive electrode can be made also with acids which are not of such type as will dissolve in water to form hydrogen ion but which have more than a certain degree of vapor pressure. For example, acetic acid gas will dissolve in water to form hydrogen ion. ##EQU9##
In the light of such reaction, an ammonia-gas sensor can be constructed which consists essentially of a gas permeable membrane, an aqueous phase including ammonium-ion containing salt or polyions, an Ag-AgCl electrode and a pH-sensitive electrode.
A sulfur-dioxide sensor may utilize an aqueous phase including sulfite-ion containing salt or polyions.
Various types of gas sensors based on the above discussed principle, where used in the medical and physiological fields and more particularly in measuring gas concentration in a living body, are employed in such a way that the sensor is inserted into a tissue of the living body. To this end, it is necessary that such a gas sensor should be microminiaturized. In other words, miniaturized pH-sensitive electrodes are needed. However, it has been recognized that miniaturization of conventional-type glass electrodes involves the following problems:
(a) The resistance of the glass membrane is limited to about 10 M.OMEGA.. Therefore, an amplifier with a higher input resistance is required.
(b) The membrane is so thin that its mechanical strength is low.
(c) Decreased electrode area will result in higher membrane resistance.
All this means that a larger and more complicated measuring apparatus is required. Moreover, the glass electrode is fragile and more liable to breakage. From the view point of practical application, these are problems with a gas sensor using a glass electrode which is designed to be inserted into a tissue of a living body for measurement of gas concentration in the body.
A CO.sub.2 sensor using a solid pH electrode of metallic oxide in place of a glass electrode is disclosed in U.S. Pat. No. 3,719,576. This electrode, smaller and more slender than a sensor using a glass electroe, is more suitable for use as such for insertion into the tissue of a living body, but it involves these difficulties:
(a) The sensor is non-flexible because of the solid electrode.
(b) Higher electric resistance due to smallness in size.
These difficulties can be eliminated by using a recently developed pH-sensitive FET transducer having a gate-insulated field-effect transistor structure instead of a glass electrode or solid electrode. A gas sensor incorporating such pH-sensitive FET transducer, as disclosed in Japanese laid-open patent application No. 53-149396, is of such construction that a pH-sensitive FET transducer and a reference electrode are housed in a tube, in spaced apart relation, and electrical insulation resin is deposited between lead-wire FET joints and the wall of the tube to stop the tube, a gas-permeable membrane being laid at the front end of the tube, with electrolyte placed in the space formed between the tube wall and the membrane. However, there are constructional difficulties with said gas sensor using the FET transducer, as follows:
(a) Miniaturization of a gas sensor requires a smaller electrolyte chamber, which means less amount of electrolyte deposited in the chamber. This makes it difficult to perform prolonged continuous measurement, if electrolyte leaks and evaporation during measuring operation are considered.
(b) The fact that the reference electrode and FET transducer are housed in the tube in spaced apart relation limits the possibility of miniaturization.
(c) The spaced-apart relation between the reference electrode and the FET transducer gives rise to greater electric resistance between the reference electrode and the FET transducer gate, with the result that noise from such sources as induction current is likely to be introduced.
A CO.sub.2 sensor designed to overcome such difficulties with the known gas sensor using the pH-sensitive transducer was proposed by T. Matsuo et al. at the 18th convention of the Japanese ME Society (May 1979). This sensor, as FIG. 2 shows, is of such construction that a pH-sensitive FET transducer 5 having a Ag-AgCl reference electrode 6 deposited thereon is placed in a glass tube 7, and electric insulation resin filling 8 is present between lead-wire-FET joints and the inner wall of the tube to stop the tube, with an aqueous solution of sodium bicarbonate placed in a space 9 defined between a gas-permeable membrane 10 and the Ag-AgCl and FET gate. However, this sensor has the disadvantage that the Ag-AgCl layer deposited on the Si.sub.3 N.sub.4 surface of the FET transducer is liable to separate from the Si.sub.3 N.sub.4 surface during measuring operation, because silver does not exhibit good adhesion with Si.sub.3 N.sub.4. This sensor is made in the following way. An Ag-AgCl reference electrode 6 is deposited on the surface of a FET transducer 5. Thereafter an aluminum layer is deposited which extends over both the gate region of the FET transducer and a part of the reference electrode. A fluoroplastic coat is placed on the surface of the aluminum layer to provide a gas-permeable membrane. Then, the aluminum layer is removed by electrolysis so that a space is formed between the fluoroplastic layer and the FET transducer gate and a portion of the reference electrode. Electrolyte is injected by means of a syringe into the space. The injection hole is stopped with slicone resin. One difficulty with this process is that the silver in the reference electrode is contaminated with aluminum as a result of the method used for forming the space. This leads to unstable signalling. Moreover, the complexity involved in the manufacturing process makes it very difficult to manufacture a reasonably reproducible sensor.
After extensive research directed for eliminating the difficulties with the CO.sub.2 sensor proposed by T. Matsuo et al., the inventors developed an improved gas sensor of novel construction.